Radio telegraph receiving system



/Pf ma@ CHAP/N Oct. 1,9, 1948. R. s. cHAPlN 2,451,915

` RADIO TELEGRAPH RECEIVING SYSTEM Filed sept. 7, y194e /f/P/ j f QF fpm1f DE? OWS /Pmcrq f #WP CO/VL/ HNP MT5/P ,/70 A70/P OSC/LL/Q/F 15A/V0/E'F f/Pfa F of? H955 f6/WW- AMP ca/vu #f7/.D LHP @fsw/vof@ f/ /c f? @07mi@ //y asc/mm? INVENTOR.

Patented Oct. 19, 1948 UNITED STAT RADIOk TELEGAPH RECEIHCIG SYSTEM`Reynold S; Chapin, Flushing, N. Y., assgnor toV Press Wireless, Inc.,New' York, N. Y., a corporation of Delaware I I. Application september7, 1946,' seial'No. 695,550

" 5 claims.

tems and more especially to such systems Wherein the telegraphinformation is represented by keyed off and on signals. I

In present day radio telegraphy, there are two generally used methods oftransmission, one identified as make and break, and the other asfrequency shift. In the make and break system, the radio carrier iskeyed-on for one signal conditi-on, e. g., mark, and is keyed-off torepresent space. In the frequency shift system, the radio carrier iscontinuously transmitted but the frequency is shifted to one valuerepresenting mark and another value representing space. One of theadvantages of the frequency shift method is an increase in thesignal-to-noise ratio as compared with the off and on system.

Accordingly, it is aprincipal object of this invention to provide an offand on radio telegraph system having certain of the advantages `of thecarrier frequency shift system, and in particular the advantage of highsignal-to-noise ratio.`

A feature of theinvention relates to a radio receiving arrangement forreceiving telegraph signals of the olf and on carrierntype, having meansto increase the ratio of signal-to-noise by automatically 'andartiically inserting `a carrier at the receiver when no carrier isbeingy received from the transmitter, as for example during thereception of a space signal.` L f i Afurther feature relates to a novelarranges ment for selectively and artificially supplying a local carrierto a radio receiver which normally depends for its signal reproductionon the receipt of a carrier from a distant transmitter. i

A still further feature relates,r to the novel organization,arrangement" and` relativev inter-Y connection of parts which cooperateto provide an improved radio telegraph receivingksystern.

Inasmuch as the various components of a vradio telegraph and receiving'Vsystem are, in

themselves, well-known, the invention will ybe illustrated in blockdiagrammatic form.` Only those parts of such a system as areinecessaryto an understanding of `the invention and one method of carrying it intopractice, are shown. The drawing shows any well-known suitable form v ofradio transmitter I of the Well-known carrier off and on type,'comprising a carrier frequency generator II which can be keyed on andoff by the telegraph keyer device I2. The keyed carrier is picked up byany suitable antenna I3, and is appliedto two radio receivers I4, I5,Ieach of which may be, although not necessarily,` ofthe superheterodynetype. Each receiver may `comprisea radio frequency amplifier stage,`afre- A'" quencyvmixer or converterstage which is suppliedwithlocal highfrequency heterodyne oscillations from the usual source II6,1II; anintermediate frequency amplifier stage, and adetector stage. Thereceiver I4 -is the main receiver whose output controls the `signalresponder device I8. Preferably, the`detect0rstage of receiver I4 isvsupplied'with local audio frequency beat oscillations fromfsource I 9,and the resultant beat note is" passed through a suitable band-passfilter 20. The receiver I5 is an auXiuiary receiver and is employed forthe purpose of supplying the intermediate frequency stage of receiver I4with an intermediate frequencycarrier when no radio frequency carrier isbeing applied via antenna I3 to the input of receiver I4. For purposesof explanation, it will be assumed that when a'mark signal is beingtransmitted by the transmitter II) a radio'frequency carrier is pickedup by the antenna I3, and when a space signal is being transmitted,nocarrier 1s picked up by the said antenna. "Connected to theintermediate frequency stage of receiver I4 is a localintermediateYfrequency oscillator 2l Vwhich Vnormally generates a frequencywithin'the pass-band of the intermediate frequency circuits of receiverI4. The arrangement is such that when no radio carrier is being receivedbythe antenna I3, the oscillator 2| is effective to apply or inject thelocal intermediate frequency into the interme- 'diate frequency stage ofthe receiver I4. On the other hand, when a radio carrier signal isibeing received by antenna I3, the arrangement Is such that theoscillator'ZI is controlled so that it does not apply any localintermediate frequency carrier to the intermediate frequency stage ofreceiver I4 within the pass-band of the said stage. It has been found`that when the receiver I4 does not receive radio carrier from theantenna I3, and in the absence of v oscillator 2|, considerable noise ispresent in the output of the receiver. On the other hand, by injectingYthe intermediate frequency oscillations artifically from `theoscillator 2| as above described, this noisecontent in the output of thereceiver I4 is greatly reduced if not entirely suppressed.

Assume for purposes of explanation that a mark signal 'is beingtransmitted. Under these conditions, a D. C. signal appears at theoutput of receiver I5 which is applied through a low-pass filter 22,inorderto increasethe ratio of signalto-noise., This D. C. signal isapplied to a reactance tube orreactance modulator 23 of any well-knowntype,rthe' output of which is coupled to the oscillator 2 I, so thatwhen a signal is passed On the other hand, when no radio frequencycarrier is being received corresponding for example to a space signal,no D. C. signal appears receiver Whose detected output controls atelegraph signal responder, an auxiliary radio receiver for producing atits output a control signal when the radio carrier is keyed-on, a localinjection oscillator for the main receiver, means responsive to thepresence of said control signal to render said local oscillatorineifective to inject .carrier into said main receiver so as to affectsaid signal responder, the last-mentioned means being effective in theabsence of said controlled signal to render said local oscillatoreffective to inject carrier into said main receiver to increase the.ratio of signal-to-noise when the radio carat the output of receiverI5. Under these condii tions, the reactance tube 23 causes theoscillator 2| to generate a sustained intermediatefrequency v carrierwhich is within the pass-band of the intermediate frequency stage ofreceiver 5M.' Preferably, although not necessarily, the injectedfrequency from oscillator 2I is slightly different fromf the normalintermediate frequency of the receiver I4,I which norm-alintermediatefrequency is produced at the output 4of the mixer stage ofreceiver I4 by heterodyning the received carrier with the localoscillator I1.; AHowever kkthe injected frequency is within the normalpass-band of this intermediate frequency stage. Thus, when the 'injectedintermediate frequency signal is detected and heterodyned with thesignal from-the local beat frequency oscillator I9, thereis produced anaudio frequency in the output of the receiver I4 which is slightlydifferent from the normal beat frequency at the output of receiver I4when a radio frequency mark signal is applied to the input of thatreceiver. When a radio frequency mark signal is being received, thereceiver I4 functions in the normal way to produce at'its output anaudio frequency beat note determined by the detected signal and the beatfrequency oscillator I9. Since the oscillator I9 is continuouslyoperating, the output of the receiver I4 for a received radio frequencymark signal will be an audio frequency tone of a specie frequency, whichis diierentfrom the tone which is created when no radio frequencycarrier is being received, but which results from the injected localcarrier. y

Preferably, the low-passlter 22 isadjustable to correlate it with thekeying speed of the received signals so as to increase a'ratio ofsignalto-noise. Likewise, if desired, the vsensitivity of the reactancetube 23 and the oscillator`2l can be adjusted to correlate them with thekeying speeds in the received signals vso as further increase the ratioof signal-to-noise.

While one particular embodiment has been disclosed, it will beunderstood' that various changes and modifications may be made thereinwithout departing from the spirit and scopeof the invention.

What is claimed is: l 1. A receiving arrangement for radio telegraphsignals of the carrier on and olf type, comprising a main radio receiverwhose output con- 2. A receiving arrangement for on and offradiotelegraph signals, comprising a. main radio rier is keyed-off.

3. A receiving arrangement according to claim 2 in lwhich said mainreceiver is continuously suppliedwith beat frequency oscillations toproduce inf-the output of said main receiver two different audiofrequency beats, one correspondingto reception of a keyed-on radiotelegraph signal and the other corresponding to the keyedoff telegraphsignal.

` 4. A receiving arrangement for on,and off radio telegraph signals,comprising a main radio receiver whose detected output controls atelegraph signal responder, an auxiliary radioreceiver, said mainreceiver being the type having an intermediate frequency carrier stage;a local intermediate frequency carrier generator, and means controlledby the detected output of said auxiliary receiver for rendering saidintermediate frequency carrierv generator effective to inject localintermediate carrier frequency into said intermediate frequency stagewithin the pass-band of said stage, the last-mentioned means beingresponsive to the reception of radio telegraph carrier to render saidintermediate frequency carrier-generator ineffective to injectintermediate frequency carrier into said main receiver within thepass-band of the said intermediate frequency carrier stage.

5. A receiving arrangement for a telegraph receiving system comprising amain signal receiving channel, an auxiliary signal receiving channel, alocal carrier oscillator for artificially injecting a carrier frequencyinto the main receiving vchannel when the received carrier is keyed off,a reactance modulator device for controlling the frequency of said localoscillator .said reactance modulator being controlled to the detectedoutput of said auxiliary receiver, said main receiver being of thesuperheterodyne type having` an intermediate frequency stage and saidlocal oscillator being arranged to generate a carrier lfrequency whichis within the pass-band of said intermediate frequency stage but isdifferent from the normal intermediate frequency of said main receiverwhen receiving keyed-on telegraph radio signalsand a beat frequencyoscillator is connected to the detector stage of said main receiver toproduce two `different .audio frequency signals correspondingyrespectively `to the reception of a keyed-on and keyed-off radiotelegraph signal.

y REYNOLD S. CHAPIN.

REFERENCES CITED The following references are of record in the le of`this patent:

UNITED STATES PATENTS Number Name Date 1,740,833 Ranger Dec. 24, 19292,125,953 Prochnow Aug. 9,1938 .2,316,017 Peterson Apr. 6, 1943

